Water harvesting from concentrated runoff for irrigation purposes in Boqueras, Spain
Water harvesting in Boqueras consists of building small earthen or stone-built bunds that divert flood water from intermittent streams towards fields cultivated with almond, orchard and/or cereals. Land users aim to achieve maximum infiltration by developing several terraces in the fields, as they reduce slope gradient and retain water for longer periods.
To prevent excessive concentration of flow, several spillways can be made per terrace, depending on the expected inflow of water. The success of this experience depends on the local knowledge of land users
Large parts of the south-east of Spain suffer from water shortage for sustainable agricultural purposes. The climate is semi-arid, with a mean annual rainfall of around 300 mm. Droughts, most common in summer, typically last for more than 4–5 months. Annual potential evapotranspiration rates larger than 1,000 mm are common. Indeed, recent decades are characterised by extreme variability of weather, causing an increase of temperature and seasonal rainfall (heavy rainfall events, both in intensity and amount) and floods, but also causing dry spells and shortening growing periods. Soils are mostly shallow to medium depth (20–60 cm), with a medium texture (loam). The infiltration rate is low and soil storage capacity is medium. Concerning the availability of conventional water resources, surface water resources are non-existent in most parts of the region, and water quality is deemed appropriate for agricultural use only. Slope grade is the most important factor of success of this experience, which in this case is gentle to moderate (5 to 15 degrees).
Conflicts arising from the baseline situation
Before implementing the water harvesting measure in Boqueras, the range of viable agricultural commodities was limited because of water scarcity and quality. Farmer incomes were low, and floods had greatly eroded soil and damaged downstream properties.
Description of the applied measure, its introduction and operation
For farmers, the first aim in implementing a water harvesting system is to increase crop yield. However, these structures also help to reduce runoff volume in intermittent streams and, consequently, they limit the intensity and damage of floods.
Land use rights are individual (most land is privately owned), but streams are not privately owned. Therefore, permits are required to construct a water harvesting structure. While water use rights are individual, they are provided and controlled by the Water Authority of the Segura River Basin (CHS). The process of setting up this measure consists primarily of identifying a suitable location for the construction of a diversion structure. Assessment of expected water inflow is needed, and this might be achieved either by means of simple field observation during rainfall events or based on the local knowledge of land users. Obviously, the quality of water could be affected by other factors, such as livestock breeding or upstream pollution, so it is important to assess the downstream implications of water harvesting.
Such structures are built (with a shovel or tractor) by creating a small bund (1.5 m height) in the centre or to the side of a stream. These structures can be strengthened with concrete to limit required maintenance to approximately once every 5 years.
Physical and ecological impacts of the measure
The measure widely improved crop yields in Boqueras and has increased soil moisture. Surface runoff was reduced and better water drainage was observed. The measure also helped to increase the infiltration rate and, consequently, recharged the groundwater table aquifer.
Financial and other impacts of the measure
The consequence of water harvesting is an increase of irrigation water availability and of crop yield, generating an increase in farm income. According to land users the practice is relatively expensive, but once installed, maintenance is not expensive and pays off because of higher productivity.
Resilience of the achievements, sustainability of the results
The adoption of the water harvesting technology is completely voluntary. At present, there is no observable trend towards its spontaneous adoption.
- Assuming an average of annual rainfall in Boqueras is 300 mm (annual rainfall varies from 250 to 500 mm/year), the practice provides up to 550 mm of additional water, resulting also in projected increases in crop yields.
- Water harvesting by building terraces reduces runoff volume in intermittent streams, which also helps to control erosion and reduce flood damage.
- Local knowledge of land users
- Easy to build: depending on the size, a bund can be built with a shovel or a tractor
- Requires monitoring and some maintenance after each important runoff event
- The volume of water available for irrigation on a given plot may increase by up to 550 mm per year.
In addition to the drought factor, the key question of replicability is the existence of gentle to moderate grades of slope (in Boqueras, the slope grade is from 5 to 15 degrees).
Workforce availability and financial supports are the most important factors in facilitating the application of this measure in other locations.
- Labour costs and price of concrete are the most determinate factors. Costs were calculated assuming bund dimensions of 5x1x1 metres. The estimated construction cost is USD 900 per hectare. Maintenance is required once every five years, so yearly costs are the total costs divided by 5. The estimated maintenance cost USD 41 per hectare per year (prices are for spring 2008).
Joris de Vente, EEZA-CSIC, Spain: Joris@sustainable-ecosystems.org
- Frot, E., van Wesemael, B., Benet, A.S. and House, M.A. “Water harvesting potential in function of hillslope characteristics: A case study from the Sierra de Gador (Almeria province, south-east Spain). Journal of Arid Environments, 72(7): 1213-1231 (2008).
- Rima Mekdaschi Studer and Hanspeter Liniger. Water Harvesting: Guidelines to Good Practice. Centre for Development and Environment. 2013. Downloaded from: https://www.wocat.net/fileadmin/user_upload/documents/Books/WaterHarvesting_lowresolution.pdf